User can place a bid on a product.

@cgruppioni @SherrySeibel  - Please take a look.  The feature is limited to being able to place the bid.  No work has been done to display the bid in the views.  New user stories are in place for those items.
What if you do `validate :other_thing, highest_bid: true` in your model? This will still add errors to the `:amount` attribute. How can you fix that?